For the filter, I would just remove the cartridge and replace it with a nylon bag if you use peat, or regular filter foam/wool, whatever.I was planning on making it a blackwater tank, using driftwood and peat moss. If I use peat moss, do I just put it in the tank or mix it in with the substrate, or do I put the peat moss in the filter, or somewhere else? If I want to dye with the peat moss and driftwood: For the filter, if the carbon is inside the blue fibre thing can I cut open the blue thing take the inside out, clean off the inside plastic and the blue thing of carbon, remove all of the carbon and put the blue thing back on with elastics?
To be clear, the RO is a completely separate unit than the tank and I filter tap water through it to get super soft neutral water that can then easily become acidic?
Don't see your tap water pH and hardness mentioned, am I correct in remembering that it is close to pH 8 and fairly hard? If yes, using peat will take a lot, and peat wears out in time and needs to be replaced, so in the filter rather than mixed in the substrate is better long-term. Driftwood will not significantly alter the pH, maybe .2 if lots of wood, but no more that I've ever heard of. RO unit would be preferable though expensive. Others can better explain RO, I've never needed it. But I think normally one uses it to treat the tap water (it removes minerals resulting in basically "pure" water with no hardness to speak of although the pH can be slightly acidic to neutral) for a water change and then mix it with some regular tap water to get the desired hardness and pH. I'll leave this for more experienced members to comment.
Once the tank has de-mineralized water (so to speak), the natural biological processes in the aquarium will cause the pH to lower. My tanks run at pH 5 or 6, because my tap water is zero GH and KH and pH is 7.0 or 7.2 but the lack of any hardness means the pH readily and quickly drops when added to the established tank. The tank at pH 6 is there because I maintain a half cup of dolomite in the filter that slightly hardens the water (it stays at GH 2) and maintains a stable pH with my weekly 50% water changes. The other two are at 5 although I plan on getting more dolomite to buffer them the same. I have mostly wild-caught acidic water fish so this works fine; my cardinals are in their element.